The present invention relates to a flame retardant resin composition, or more particularly, to a flame retardant polypropylene resin composition which comprises polypropylene having improved flow melt characteristics, flame retardants, a flame retardant aid, and a tetrafluoroethylene polymer. The resin composition of the present invention has high melt tension, without deterioration of the mechanical properties of flame retardant polypropylene, and drastically enhanced characteristics of shape maintenance and flaming drip during burning.
As for the conventional resins used in electrical products, non-combustible, flame-retardant, or self-extinguishing resins are used for the purposes of preventing ignition, combustion or fire, which may be caused by various electrical troubles therein. In this regard, since olefin-based resins made from hydrocarbons(i.e., polypropylene) have excellent impactability, rigidity, appearance and moldability, various proposals have been made relating to flame retardancy of these resins. In particular, according to the standards of flame retardancy of electrical products, the UL specifications (UL94) of the US require a high level of flame retardancy, depending on the products and the parts. The products to be exported to the US are required to be made from materials deemed to be appropriate according to the UL specifications. In response to these requirements, researchers have developed a material of thermoplastic polyolefin resins, mixed with organic and inorganic flame retardants, and a flame retardant aid. However, the material is problematic in that it suffers from the occurrence of a flaming drip. To solve this problem, a flame retardant resin composition was proposed, wherein said composition comprises 30xcx9c80 wt % of polypropylene; 5xcx9c25% of polyethylene having a melt flow rate of 0.01xcx9c2.0 g/10 minutes (190xc2x0 C., 21.18N); 10xcx9c35 wt % of an inorganic fillers selected from the group consisting of powder talc, kaoline, mica, and silica; and 3xcx9c35 wt % of decabromodiphenylether and/or dodecachloro decahydrodimetabibenzocyclocutene (Japanese Patent Publication No. Sho 55-30739). In the above patent, it was reported that polyethylene acts in effect to facilitate the melt flow drip therein during burning. However, with respect to the composition as such, if the melt flow rate (MFR, 190xc2x0 C.; 21.18N) of polyethylene is low, there is a problem of insufficiency in even dispersion of polypropylene-based flame retardant resins. For this reason, an increase in melt tension is not seen, with insubstantial effects on the improvement of the properties of shape maintenance and a flaming drip. Consequently, if the amount of polyethylene in the composition is 5 wt % or less, a proper melt-drip characteristic is not obtained. Conversely, if the amount in the composition increases, the temperature of thermal deformation, rigidity, etc. deteriorate. Moreover, the superior characteristics of polypropylene are lost.
Moreover, as for the method of increasing melt tension or the temperature of crystallization of a polypropylene composition, there are several methods as follows: a method of reacting organic peroxides and cross-linking agents to crystalline polypropylene in the molten state (Japanese Patent Laid-Open Nos. Sho 58-93711, Sho 61-152754, etc.); and a method of producing polypropylene having free monomer chain branches without gel by reacting semi-crystalline polypropylene with a peroxide of a low decomposition temperature in the absence of oxygen (Japanese Patent Laid-Open No. Pyung 2-298536). In addition, as for the other methods of enhancing melt visco-elasticity (e.g., melt tension), researcher have proposed a composition of a mixture of polyethylene or polypropylene of different specific viscosities or molecular weights, or a method of obtaining such a composition by means of multi-step polymerization. For example, the following methods have been proposed: a method of extrusion in the temperature range of the melting point to 210xc2x0 C. after adding 2xcx9c30 weight parts of polypropylene of ultra-high molecular weight to 100 weight parts of commonly-used polypropylene (Japanese Patent Publication No. Sho 61-28694); a method of multi-step polymerization, yielding an extrusion sheet which comprises two components of polypropylenes of different molecular weights, having a limiting viscosity number ratio of 2 or higher (Japanese Patent Publication No. Pyung 1-12770); a method involving a polymer of 1xcx9c10 wt % of polyethylene of low viscosity; a method of melt-mixing three types of polyethylenes of different viscosities and molecular weights; a method of multi-step polymerization of polyethylene of ultra-high molecular weight having limiting viscosity number of 20 dl/g or higher, wherein less than 0.05xcx9c1 wt % of the polyethylene is obtained(Japanese Patent Publication No. Pyung 5-79683); and a method of multi-step polymerization of less than 0.05xcx9c1 wt % of polyethylene of ultra-high molecular weight having limiting viscosity number of 15 dl/g or higher by means of using 1-butene or 4-methyl by way of a polymerizer of a specific alignment, wherein (Japanese Patent Publication No. Pyung 7-8890).
With respect to the various compositions or the methods of preparation thereof as proposed in prior art, those methods enhance melt tension of polyolefin to a certain extent. However, there are problems of inadequacy of recycling usability with those cross-linked compositions in addition to the problems of low rigidity at high temperature. Furthermore, with respect to those using polyethylene of high viscosity, there are many factors which need improvements: e.g., an increase in electricity consumption due to an increase in current load at the motor of a molding machine, limitation in productivity, and low thermal stability. Moreover, with respect to the method of preparing polyolefin polymers of high molecular weights, according to said method of multi-step polymerization, it is difficult to control a low polymerization level in olefin (co)-polymerization for producing a small amount of polyolefin of high molecular weight In addition, it requires low polymerization temperature for producing polyolefin of sufficiently high molecular weight. The method also requires reconstruction of the process, which all leads to lowering of productivity of polyolefin.
In the end, with respect to the prior art as mentioned above, it brings about insufficient enhancement in terms of melt tension of polypropylene and its crystallization temperature. In particular, a composition having superior rigidity and moldability has not been developed as of yet, which simultaneously satisfies the characteristics of shape maintenance, and a flaming drip or melt drip at the time of burning of a flame retardant polypropylene resin composition comprising a halogen-based flame retardant.
In solving these problems as mentioned above, the objective of the present invention lies in providing a flame retardant polypropylene resin composition having improved characteristics of shape maintenance and a flaming drip.
The flame retardant polypropylene resin composition of the present invention comprises (A) a polypropylene resin having a melt flow rate (hereinafter MFR) of 1.5xcx9c40 g/10 minutes at 230xc2x0 C. and a 2.16 kg load; (B) at least one of flame retardants and a flame retardant aid; and (C) 0.10xcx9c3 wt % of a tetrafluoroethylene polymer.
With respect to the flame retardant polypropylene resin composition of the present invention, the polypropylene resin refers to a propylene xcex1-olefin block copolymer having 50 wt % or more of propylene polymer units or a propylene homopolymer. In other words, the polypropylene resin used in the present invention refers to a crystalline polypropylene homopolymer, or a copolymer of propylene and one or more compounds selected from the group consisting of 1-butene, 1-pentene, 1-hexene, 4-methylpentene, 1-heptene, 1-octene, and 1-decene. The melt flow rate of the polypropylene resin should be 1.5xcx9c40 g/10 minutes, or preferably 2xcx9c40 g/10 minutes. If the melt flow rate is less than 1.5 g/10 minutes, the injection molding of a thin product becomes rather difficult, which is not preferable. If the melt flow rate exceeds 40 g/10 minutes, the melt viscosity is low, and consequently it does not display the drop preventing effect of the molten material, which is characteristic due to tetrafluoroethylene. In this regard, a V0 grade of flame retardancy according to the UL specifications cannot be obtained.
With respect to the stereo-specificity of the polypropylene resin used in the present invention, it can be of any polypropylene without specific limitations as long as it is crystalline. In particular, a crystalline polypropylene resin can be used, wherein its isotactic pentad ratio measured with 13C-NMR (Nuclear Magnetic Resonance Spectrum) is 0.80xcx9c0.99, or preferably 0.85xcx9c0.99. In particular, a crystalline polypropylene resin of 0.90xcx9c0.99 of isotactic pentad ratio can be preferably used.
With respect to the flame retardant polypropylene resin composition of the present invention, at least one of flame retardants is selected from the group of inorganic compounds such as magnesium hydroxide, hydrotalcite, and ammonium polyphosphate, etc.; and the group of halogenated compounds such as decabromodiphenylether, ethylene-bis(tetrabromophthalimide), bispentabromophenoxyethane, tetrabromo bisphenyl A-bis(2,3-dibromopropylether), bis{(3,5-dibromo-4-(2xe2x80x2,3xe2x80x2-dibromopropyloxy))phenyl)}sulfone, etc. For the purpose of achieving the effect of flame retardancy, the amount of flame retardants is preferably 10xcx9c30 wt % of the composition.
With respect to the polypropylene resin composition of the present invention, commonly-used flame retardant aids may be used, or preferably antimony oxide can be used. For example, antimony trioxide, or antimony pentoxide or the mixture thereof can be used. For the purpose of achieving the effect of flame retardancy, the amount of the flame retardant aid is preferably 5xcx9c10 of the composition.
With respect to the flame retardant polypropylene resin composition of the present invention, an appropriate tetrafluoroethylene polymer is a polymer containing 65xcx9c76 wt %, or preferably 70xcx9c76 wt %, of fluorine. In addition to the homopolymers of tetrafluoroethylene, for the same purpose as mentioned above, one can use a copolymer of other monomers containing fluorine and tetrafluoroethylene, or a copolymer of tetrafluoroethylene and ethylenically unsaturated monomers, capable of copolymerization, without fluorine. The tetrafluoroethylene polymer may be present to the extent of 0.10xcx9c3 wt %, or preferably 1.5xcx9c2 wt % of the resin composition of the present invention. Upon addition of this component therein, tetrafluoroethylene exists in the fibril form during the molding process of the resin composition consequently it has the effect of preventing molten materials from dropping, at the time of the burning of the molded materials As for the tetrafluoroethylene polymer used in the present invention, it is preferable to use a polymer in the form of powder or solid. The method of producing a tetrafluoroethylene polymer is well know as proposed for example in Houden-Wey, Metrodender Organischem Chmie, Volum 14/1, pg. 842xcx9c849 (Stuttgart 1961). The addition of a tetrafluoroethylene polymer in the composition according to the present invention reduces the amount of flame retardants necessary therein and consequently it enhances the mechanical properties of the products produced from the composition of the present invention. Moreover, by adding a tetrafluoroethylene polymer, it prevents molded bodies from burning during dripping of the combustible particles. For obtaining the effect of flame retardancy therein, the amount of a tetrafluoroethylene polymer is preferably 0.10xcx9c3 wt %, and the amount of fluorine contained in the tetrafluoroethylene polymer is preferably 65xcx9c76 wt %. If the amount of fluorine in tetrafluoroethylene polymer is less than 65 wt %, or more than 76 wt %, the effect of flame retardancy is not quite sufficient.
In addition to the above components, within the scope not deviating the purposes of the present invention, conventional additives such as inorganic fillers and antioxidants can be added to the flame retardant polypropylene resin composition.
The present invention is described in detail by examples as follows.
The assessment for flame retardancy in the present invention was based on the vertical burning tests carried out along vertically according to the xe2x80x9cTests for Flammability of Plastic Materials for Parts in Devices and Appliancesxe2x80x9d of UL Subject 94 (Underwriters Laboratories, Inc.). The thicknesses of the test section pieces used therein were {fraction (1/16)} and {fraction (1/32)} inch, respectively.
The compositions produced by the present invention were classified as UL94 V0 when they satisfied the following standards: A set of five test pieces with respective measurements of 127xc3x9712.7xc3x971.6 mm was placed directly in contact with the flame (height of 19 mm). There were two contacts, respectively, which were maintained for ten seconds. Then, after ten or more seconds of flame burning, none of the test pieces should burn. For applying the flame to the set of five test pieces for ten times, the total duration of flame burning must not exceed 50 seconds. None of the test pieces should drop flame particles. None of the test pieces should burn in flame to the point of the clamp""s grip or burn further thereafter. After removing the test flame for the second time, none of the test pieces should undergo sustained burning for more than 30 seconds.
To be classified as UL94 V1, the duration of flame burning per test piece must not exceed more than 30 seconds. Moreover, with respect to applying the flame to the set of five test pieces for ten times, it requires that the total duration of flame burning to be not longer than 250 seconds. There, the burning must not sustain for more than 60 seconds. Other standards are the same as the conditions as mentioned above.
Those test pieces dropping flame particles while satisfying the other standards for classifying them to UV94 V1 were classified as UL94 V2.